The present invention generally relates to a sensing instrument to evaluate a mechanical force. More particularly, the present invention relates to a micro-sensing device which can be utilized in biological and non-biological applications to infer local stiffness and other dynamic parameters, for example, measurement of mechanical impedance and/or variation of impedance with time or other conditions, from which, for example, the state of osseointegration of a dental implant may be deduced.
Optimal treatment using Branemark-style endosseous dental implants requires that the implant be stably osseointegrated before it is loaded. Research has shown that measurements of mechanical impedance can be used to infer the general state of osseointegration. However, to date, the accomplishment of such a measurement requires that the implant be surgically exposed before it can be tested utilizing a hand-held probe. Examples of prior art devices for measuring the osseointegration of bones are well known in the art. Particular examples include U.S. Pat. No. 5,024,239 to Rosenstein which discloses an apparatus for detecting loosening of an implant embedded in a bone of a limb of a patient. The disclosed apparatus includes a vibrator which is pressed into engagement with a limb to impart a vibratory motion to the bone which is received by a pickup device which is pressed into engagement with the limb a distance from the vibrator wherein the output signal transmitted through the bone implant is analyzed in order to measure the amount of implant fixation. However, the Rosenstein ""239 patent does not disclose an implantable or partially implantable apparatus.
U.S. Pat. No. 5,518,008 to Cucchiaro et al. discloses a dental analyzer for analyzing dental implants which includes a dental probe which is placed in contact with a patient""s dental implant. A force is applied to the dental implant through a hammer fired by an actuator disposed in the probe which impacts the dental implant and vibrates the dental implant. An accelerometer, disposed within the probe, measures the acceleration time history of the vibrating implant and a processor converts the measured acceleration time history into a measurement indicative to the condition of the dental implant. However, the Cucchiaro et al. ""008 patent does not disclose an implantable or partially implantable device which is utilized to assess the mechanical impedance of the surrounding physical structures including human or animal tissues.
U.S. Pat. No. 4,754,763 to Doemland discloses a non-invasive method of testing the integrity of an in vivo bone and an apparatus for performing the method which includes placing an electrical vibration transducer against the exterior of the soft tissue surrounding the bone and generating a mechanical vibration in the bone by striking the bone with a reflex hammer. The signal received by the vibration transducer is converted into a measurement of the integrity of the bone. However, the Doemland ""763 patent does not disclose an implantable or partially implantable device for measuring the mechanical impedance of physical structures.
All of the prior art devices for either sensing and measuring the degree of osseointegration or the measurement of local stiffness utilize external probes. It would be preferable and desirable if a clinician could evaluate the degree of osseointegration in a non-invasive manner, exposing the implant only when it is demonstrably stable enough for loading. Additionally, it would be further desirable and advantageous to have a micro-sensing device whose primary sensing components are small enough that they can be temporarily or permanently implanted, for example, as a component of a dental implant. Further, a device embodying this invention also has utility to temporarily or permanently be emplaced within non-living materials or structures whose condition or integrity could thereby be nondestructively monitored on the basis of local mechanical impedance.
A dynamic sensing device includes a force generator, an accelerometer unit, an electronic unit, a power source and a control signal source. The electronic unit calculates a force measurement from an output received from the force generator or accelerometer unit. The dynamic sensing device is sufficiently small as to be implantable within a mechanical structure or biological tissue. A force sensor is also included when the force generator does not yield a force with a known time profile.
An implant according to the present invention includes a housing having a cavity therein, a closure adapted to selectively engage the cavity of the implant housing, and a force sensing device as detailed above. The use of an implanted device to produce a force independent of a preload bias in order to measure impedance of an implant is also disclosed.
A process is disclosed for applying a known force at the boundary between the device and a structure to be characterized, without a concomitant preload bias. By sensing the acceleration of a secondary mass elastically mounted to the device, the acceleration of the device is readily calculated. By a mathematical combination of the applied force and resultant acceleration, it is possible to determine the impedance of the structure to be characterized, which impedance is often a parameter of diagnostic interest.